Protective system for automatic actuation of steam turbine drain valves

ABSTRACT

System for automatically opening drain valves to drain condensate from a steam turbine when the load on the turbine falls below a predetermined limit beyond which condensate within the turbine casing is likely to occur. Preferably, load conditions are sensed by a pressure switch actuable in response to a low pressure-low load condition within the turbine casing. This switch then closes electrical circuitry for opening the drain valves when the pressure falls below the aforesaid predetermined limit.

United States Patent Cohen et al.

1 1 Sept. 26, 1972 [54] PROTECTIVE SYSTEM FOR AUTOMATIC ACTUATION OF STEAM TURBINE DRAIN VALVES Primary Examiner-Henry F Raduazo Attorney-A, T. Stratton, F. P. Lyle and F. Cristiano, Jr.

[72] Inventors: Albert Cohen; Frank O.

Burckhalter, both of Wallingford, [57] ABSTRACT System for automatically opening drain valves to drain 7 Assignee; Westinghouse Ekctric Corporation condensate from a steam turbine when the load on the Pittsburgh, p turbine falls below a predetermined limit beyond t which condensate within the turbine casing is likely to [22] Filed 1971 occur. Preferably, load conditions are sensed by a [21] Appl. No.: 114,000 pressure switch actuable in response to a low pressurelow load condition within the turbine casing. This 52 us. c1. ..415/168, 415/219 R Switch elses electrical circuitry Pening 51 1111.01. ..F0ld 1/00, FOld 25/24 drain "eh/es when the Pressure falls helew the [58] Field of Search ..4l5/l68, 121, 219 R efereeeld Predeemmed [56] References Cited UNITED STATES PATENTS 6 Claims, 1 Drawing Figure 2,004,777 6/1935 Bassler ..4l5/2l9 R COMPRESSED SU PPLY PROTECTIVE SYSTEM FOR AUTOMATIC ACTUATION OF STEAM TURBINE DRAIN VALVES BACKGROUND OF THE INVENTION It sometimes happens that a steam turbine is damaged because of the accumulation of water in the base of the turbine casing. This water acts to cool the base, creating high thermal gradients which distort the casing. As a result, a rub usually occurs and causes extensive damage.

The accumulation of condensate within the turbine casing does not occur unless the load on the turbine is below a predetermined level. This is probably due to the fact that under usual load conditions, extensive amounts of steam pass through the turbine and supply heat to the casing and other surrounding parts. Under reduced load conditions, however, the steam input is reduced; the heat supplied to the turbine casing is reduced also; and the casing cools such that steam can condense and collect at the bottom. Normally, excessive condensation occurs when the turbine is operating at or below about 20 percent of full load conditions.

Operating personnel for turbine installations are instructed to open the drain valves at the bottom of the turbine from zero speed up to 20 percent of maximum load. However, this practice is not observed at all times with the result that extensive damage to the turbine can result. This is particularly true during a trip out when the operator is concerned with other emergency functrons.

SUMMARY OF THE INVENTION In accordance with the present invention, damage to the turbine because of accumulation of water in the turbine casing under part load conditions is avoided by providing a system which automatically senses that the load on the turbine is below a predetermined value and automatically opens the drain valve or valves at the bottom of the turbine casing whenever the load falls below that value.

Specifically, there is provided a steam turbine casing structure, a rotor within the casing structure, and at least one drain at the bottom of the casing structure for draining condensate from the interior thereof. A valve device, which is normally closed, controls the discharge of condensate through the drain. Means are provided for sensing a condition where the load on the turbine is under a predetermined limit below which condensate is likely to collect within the casing structure. Finally, actuating means are coupled to the sensing means to open the valve device when the load on the turbine falls below the aforesaid predetermined limit to drain condensate from the casing structure.

In the preferred embodiment of the invention, the means for sensing when the load on the turbine is below a predetermined limit comprises a pressure switch responsive to the pressure within the turbine casing itself. Since the pressure within the casing is a function of the load on the turbine, this can be used to determine when the load falls under the minimum value below which the drain valves should be opened.

In the embodiment of the invention shown herein, the valves are air-actuated and the pressure switch is used to close an electrical circuit which admits'air to the drain valves, causing them to open. It should be understood, however, that other and different types of valve actuating means can be used. For example, instead of sensing the magnitude of the load with a pressure switch, it can be sensed by other means. If the turbine is coupled to a generator, load conditions below a predetermined value can be determined by sensing the power output from the generator or by sensing the torque on the turbine shaft.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single FIGURE drawing which schematically illustrates one embodiment of the invention.

With reference now to the drawing, a steam turbine is schematically shown and includes an outer casing 10 and an inner casing 12. Within the inner casing 12 is a rotor 14 shown partially in cross section and connected to a main shaft 16. The rotor 14 is supported in suitable bearings, not shown.

Steam in an input line 18 is supplied to the turbine through a steam chest 20 and valving, not shown, to an annular nozzle chamber 22 formed in the inner casing 12. From the chamber 22, the steam passes through nozzles 24 and thence through first stage blading 26 and second and third stage blading 28 and 30, respectively, before passing to a steam exhaust hood 32 and thence to a condenser, not shown, The second and third stage blading 28 and 30 are carried on blade rings 34 and 36, respectively, which are separated one from the other.

As was explained above, it is highly undesirable in a turbine of this type to have water collect at the bottom of the inner casing 12, such as water collecting in chambers 38 beneath the blade rings 34 and 36. However, water will accumulate under part load conditions, particularly when the load drops below 20 percent of maximum load. When this occurs, less steam is being supplied to the turbine; less heat is also being supplied to the inner casing 12; and as a result the inner casing cools and condenses steam which collects in the chambers 38. This water causes a cooling of the lower portion of the inner casing 12 with the result that it becomes distorted due to thermal stresses. This can result in a rub, (i.e., interference between the rotor and stationary blading due to loss of uniform running clearance) and extensive damage to the turbine.

The chambers 38 are provided with drain outlets 40, each of which is provided with a valve 42, 44 or 46. In the past, the valves 42-46 were manually operated; and operating personnel were instructed to open the valves when the load fell below 20 percent of full load. Failure to follow this operating procedure, however, can result in extensive damage to the turbine as mentioned above.

The present invention provides a means for automatically opening the valves 4246 when the load on the turbine drops below a predetermined minimum value such as, for example, 20 percent of full load. The valves 4246 are actuated by means of pneumatic cylinders 48-52, respectively; however, it should be understood that electrical or some other type of actuating means can be used in the case where a source of compressed air is unavailable. One end of each of the cylinders 48-52 is connected through an associated valve 54, 56 or 58 to a compressed air supply 60.

Under normal operating conditions with the load on the turbine above percent of the full load, the valves 54-58 will be closed as will be the valves 42-46. However, should the load drop below 20 percent of full load, the pressure within the inner casing 12 will drop. This can be sensed, for example, by a pressure switch 62 having a connection to one of the chambers 38. The pressure switch 62, of course, could be connected to other points within the inner casing 12. When the pressure switch 62 closes, an electrical circuit is completed to three solenoid actuators 64 which, in turn, move the valves 54-58 to the open position. Opening of valves 54-58 pressurizes one end of each of the cylinders 48-52, causing the pneumatically operated valves 42-46 to open, whereby any condensate at the bottom of the inner casing 12 can drain out. When the turbine load again exceeds a predetermined minimum value such as 20v percent of full load, this condition will again be sensed by the pressure switch 62. That is, as the steam pressure in chamber 22 rises to reflect the load increase, the pressure switch 62 will open; solenoids 64 will become deenergized; and valves 54-58 and 42-46 will close. A manually operated switch 66 is provided in shunt with the switch 62 to manually complete the circuit through the solenoids 64 and open the valves 42-46, if desired.

Instead of determining the load on the turbine by sensing the pressure within the turbine casing, it is also possible to sense the load by external load sensing means, schematically indicated by the reference numeral 68 in the drawing. This external load sensing means may comprise a device for sensing the electrical power delivered at the output of an electrical generator driven by the turbine through the shaft 16 or a ponybrake arrangement connected to the turbine shaft 16. In either case, the result will be to close contacts 70 to complete the circuit through the solenoids 64, whereupon the valves 42-46 will open to drain any condensate from the interior of the turbine casing at low load conditions.

Although the invention has been shown in connection with certain specific embodiments, it will be readi- 1y apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention. In this respect, it will be apparent that the invention can also be used to automatically drain condensate from the inlet and reheat pipes connected to the turbine, as well as the turbine casing itself.

We claim as our invention:

1. In a steam turbine, having a rotor encircled by a casing through which steam passes, at least one drain at the bottom of said casing for draining condensate from the interior thereof, normally closed valve means for controlling the discharge of condensate through said drain, means for sensing a condition where the load on said turbine falls below a predetermined limit at which condensate is likely to collect within said casing, and means incorporating electric circuit means coupled to said sensing means for opening said valve means when the load on the turbine falls below said predetermined limit to drain condensate from said casing.

2. The combination of claim 1 wherein the means for sensing a condition where the load onsaid turbine falls under a predetermined limit below which condensate 1S likely to collect comprises a pressure switch responsive to the pressure within said casing structure.

3. The combination of claim 2 wherein said valve means is pneumatically operated and said electrical circuit means includes solenoids for admitting air to said pneumatically operated valve means to actuate the same.

4. The combination of claim 1 wherein there is a plurality of drains connected to said casing structure, and normally closed valve means in each of said drains.

5. The combination of claim 1 wherein the means for sensing the load on said turbine comprises means for determining the torque on the turbine shaft.

6. The combination of claim 1 wherein the means for sensing the load on said turbine comprises means for determining the power output of an electrical generator driven by said turbine. 

2. The combination of claim 1 wherein the means for sensing a condition where the load on said turbine falls under a predetermined limit below which condensate is likely to collect comprises a pressure switch responsive to the pressure within said casing structure.
 3. The combination of claim 2 wherein said valve means is pneumatically operated and said electrical circuit means includes solenoids for admitting air to said pneumatically operated valve means to actuate the same.
 4. The combination of claim 1 wherein there is a plurality of drains connected to said casing structure, and normally closed valve means in each of said drains.
 5. The combination of claim 1 wherein the means for sensing the load on said turbine comprises means for determining the torque on the turbine shaft.
 6. The combination of claim 1 wherein the means for sensing the load on said turbine comprises means for determining the power output of an electrical generator driven by said turbine. 